Contamination detection and quantification requirements have become increasingly important, particularly with the rapid evolution of high-tech industries. For example, the semiconductor industry has developed technology for precisely producing microelectronic devices. In order to reliably produce such products, highly stringent contamination standards must be maintained in the production facilities.
In an effort to control and minimize contamination in crucial stages of a production process, “cleanrooms” are frequently used. A cleanroom is a room in which the air filtration, air distribution, utilities, materials of construction, equipment, and operating procedures are specified and regulated to control airborne particle concentrations to meet appropriate airborne particulate cleanliness classifications.
It is important to monitor the cleanliness/contamination levels in a cleanroom, especially for detecting particles on a cleanroom surface. Visual inspection techniques have been used with ultraviolet or oblique white light. Ultraviolet light is employed to take advantage of the fact that certain organic particles fluoresce. Alternatively, white light is shined towards the test surface at an angle so as to produce reflections that can be visualized. While the white light technique is slightly more sensitive than the ultraviolet technique, they both suffer from the same limitations. These visual inspection techniques only allow a cursory inspection of the surface conditions. They do not provide quantitative data. Also, the visual inspection techniques, at best, only detect particles that are larger than twenty microns. It is often desirable to detect particles that are less than one micron.
Another inspection technique involves removing particles from a test surface, by for example, applying a piece of adhesive tape to the test surface. The particles on the tape are then manually quantified by putting the tape under a microscope and visually counting the particles. This technique allows the detection of particles of approximately five microns or larger. The primary disadvantage of this technique is that it is very time consuming, and that it is highly sensitive to variability between operators.
A third inspection technique is disclosed in U.S. Pat. No. 5,253,538, which is expressly incorporated herein by reference. The '538 patent discloses a device that includes a scanner having at least one opening for receiving particles from the sample surface. The scanner is connected to a tube having first and second ends. The first end of the tube is connected to the scanner and the second end of the tube is connected to a particle counter that employs optical laser technology. The particle counter includes a vacuum generator that causes air to flow from the sample surface through the scanner, through the tube and into the particle counter, where particles contained in the air stream are counted. The '538 patent discloses an inspection method that involves the use of the particle counting device. A background particle level of zero is first established by holding the scanner near the cleanroom supply air and taking repeated readings, or by installing an optional zero-count filter in the particle counter. Next, the hand-held scanner is passed over the sample surface at a constant rate for a predetermined test period. The test cycle is started by pushing the run switch, which is located on the scanner. The particle counter counts and reads out a number corresponding to the average number of particles per unit area. The process is usually repeated several times along adjacent surface areas, each time yielding a “test reading”.